This invention relates to a catalyst and a method for converting an oxygenate feedstock to an olefin product. In particular, this invention is directed to a method for converting an oxygenate feedstock to an olefin product by contacting the feedstock with an aluminophosphate (ALPO) bound silicoaluminophosphate (SAPO) catalyst containing iron, cobalt and/or nickel, which can be tailored to optimize its performance.
Olefins have traditionally been produced through the process of petroleum cracking. Because of the limited availability and high cost of petroleum sources, the cost of producing olefins from such petroleum sources has the potential to steadily increase. Light olefins such as ethylene and propylene serve as feeds for the production of numerous chemicals and polymers.
The search for alternative materials for the production of light olefins such as ethylene and propylene has led to the use of oxygenates such as alcohols, and more particularly to methanol and ethanol or their derivatives as feedstocks. These and other alcohols may be produced by fermentation or from synthesis gas. Synthesis gas can be produced from natural gas, petroleum liquids, carbonaceous materials including coal, recycled plastics, municipal wastes, or any organic material. Thus alcohols and alcohol derivatives may provide non-petroleum based routes for hydrocarbon production.
Silicoaluminophosphates (SAPOs) are structured crystalline molecular sieves which have found application as catalysts. In particular, the use of SAPOs in converting alcohols or ethers to olefin products, particularly ethylene and propylene, is becoming of greater interest for large scale, commercial production facilities.
In contrast to SAPOs, an aluminophosphate (AlP4) framework inherently is neutral in electrical charges. The incorporation of silicon or other metallic or nonmetallic elements into the framework by substitution generates more active catalytic sites, particularly acid sites, and increased acidity. Thus, non-acidic aluminophosphates (ALPOs) lack silicon or another substituent that generates acidic charges.
Oxygenates are a promising alternative feedstock for making light olefins. Particularly promising oxygenate feedstocks are alcohols, such as methanol and ethanol, dimethyl ether, methyl ethyl ether, diethyl ether, dimethyl carbonate, and methyl formate. Many of these oxygenates can be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any appropriate organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for light olefin production. SAPOs and zeolites are catalysts known to convert oxygenates to light olefins.
With regard to zeolites, for example, U.S. Pat. No. 3,244,766 to Keough teaches a process for ethanol conversion to ethylene with a hydrogen exchanged mordenite catalyst. Keough discloses that such catalyst allows the dehydration of alcohol to occur at temperatures below 300xc2x0 C.
U.S. Pat. No. 4,025,576 to Chang et al. teaches a multistage process for converting light alcohols to olefins using certain crystalline zeolites having a high silica:alumina ratio and a constrained access to the crystalline space, for example HZSM-5. Cheng et al. indicate that a critical feature of the invention, namely conducting the conversion from alcohol to olefin at subatmospheric partial pressure of the reactant feed and using certain crystalline zeolites, enhances selectivity for olefin production and permits complete conversion of the alcohol.
Zeolite-bound zeolites, also described as binderless zeolites, have also been used to convert oxygenates to olefins. U.S. Pat. No. 5,460,796 to Verduijn et al. discloses a substantially binderless zeolite produced by an aging process that substantially converts the silica binder to zeolite having mechanical strength that is comparable to and perhaps stronger than that of silica-bound zeolite aggregate.
SAPOs are also known as being useful in methanol to olefin conversions. For example, U.S. Pat. No. 4,499,327 to Kaiser et al. discloses a process for converting methanol to olefins (MTO) using non-zeolitic molecular sieves such as SAPO-34.
One problem with SAPO catalysts is the presence of acid sites on the external surface thereof that are not shape selective. Thus, the external acid sites can adversely affect product yields. Further, amorphous (non-molecular sieve) binders that are frequently used to bind SAPO molecular sieves are thought to reduce the access of the pores of the SAPO during the conversion of oxygenates to olefins. A need exists in the art for a catalyst that retains the favorable aspects of SAPO catalysts but that eliminates the problems caused by the external acid sites on the catalyst and pore accessibility.
Moreover, in converting a feedstock containing an oxygenate to a product including an olefin, better selectivity to product, as well as away from undesirable by-product, is still needed. It is particularly desirable to obtain product high in ethylene and/or propylene content, while reducing the amount of any one or more of the C1-C4 paraffin by-products and to reduce the amount of coke deposits on the catalyst during the reaction.
In order to overcome various problems presently inherent in the art, this invention provides various embodiments of a catalyst and a method for continuous production of olefin product from an oxygenate-containing feedstock. In one embodiment, a catalyst comprises SAPO crystals, a binder comprising ALPO crystals, and nickel, cobalt and/or iron, wherein the catalyst does not contain significant amounts of amorphous binder, but rather contains crystalline ALPO. The catalyst preferably contains less than 10% by weight of amorphous binder based on weight of the SAPO and the ALPO, more preferably less than 5%, and most preferably the catalyst is substantially free of amorphous binder.
The catalyst is prepared by a process comprising preparing a SAPO framework; removing a template from the SAPO framework; preparing an alumina-bound SAPO from the detemplated SAPO framework and amorphous alumina; converting the amorphous alumina to crystalline ALPO to provide an ALPO-bound SAPO; and incorporating iron, cobalt, and/or nickel into the catalyst after the removing step or after the converting step.
In another embodiment, the method of making olefins comprises providing an ALPO bound SAPO catalyst containing iron, cobalt and/or nickel; and contacting the catalyst with the feedstock containing an oxygenate under conditions effective to convert the feedstock containing an oxygenate to a product including an olefin. In a particular embodiment, the invention is directed to an olefin prepared by this process. Such olefins can be further processed by contacting the olefin product with a polyolefin-forming catalyst under conditions effective to form a polyolefin. Alternatively, such olefins can be recovered from the product to prepare an olefin derivative preferably selected from the group consisting of aldehyde, alcohol, acetic acid, linear alpha olefins, vinyl acetate, ethylene dichloride, vinyl chloride, ethyl benzene, ethylene oxide, cumene, isopropyl alcohol, acrolein, allyl chloride, propylene oxide, acrylic acid, ethylene rubber, propylene rubber, acrylonitrile, a dimer of ethylene, propylene or butylene, and a trimer of ethylene, propylene or butylene.
Iron, cobalt and/or nickel is incorporated into the SAPO portion of the ALPO bound SAPO. The iron, cobalt and/or nickel containing ALPO bound SAPO catalyst is of great benefit in large scale commercial processes of making olefin product from oxygenate feedstock, particularly making olefins containing ethylene or propylene from feedstock comprising methanol or dimethyl ether. The presence of iron, cobalt and/or nickel increases the selectivity of ethylene and/or propylene in comparison with SAPO and reduces the byproducts formed during the olefin conversion process. Thus, the catalyst of the present invention finds particular application in oxygenate conversion processes where catalyst acidity in combination with crystalline structure, as well as high selectivity for ethylene and/or propylene, are important for reaction selectivity.
In the embodiments, the SAPO crystals have an average particle size greater than about 0.1 micron, preferably 0.1 to about 15 microns, and more preferably 1 to 6 microns. Preferably, the ALPO crystals have an average particle size that is less than the SAPO crystals. The ALPO crystals preferably have an average particle size of less than 1 micron, and more preferably about 0.1 to about 0.5 microns. The ALPO crystals are intergrown and form at least a partial coating on the SAPO crystals.
In the embodiments, the silicoaluminophosphate molecular sieve is preferably selected from the group consisting of SAPO-5, SAPO-8, SAPO-11, SAPO-16, SAPO-17, SAPO-18, SAPO-20, SAPO-31, SAPO-34, SAPO-35, SAPO-36, SAPO-37, SAPO-40, SAPO-41, SAPO-42, SAPO-44, SAPO-47, SAPO-56, metal containing forms thereof, intergrowths and mixtures thereof, and preferably SAPO-17, SAPO-18, SAPO-34, SAPO-35, SAPO-44, SAPO-47 and mixtures and intergrowths thereof.
In the embodiments, the aluminophosphate coating is preferably selected from the group consisting of ALPO-41, ALPO-5, ALPO-11, ALPO-17, ALPO-18, ALPO-19, and ALPO-34, and more preferably ALPO-17, ALPO-18, ALPO-11, ALPO-5, and ALPO-34. Preferably, the ALPO crystals are present in an amount in the range of from about 10 to about 60% by weight based on the weight of the SAPO. The ALPO crystals are not acidic, which enhances the performance of the catalyst. Further, the structure type of the non-amorphous ALPO crystals can be selected so that it does not significantly adversely affect the reactants exiting the pores of the SAPO. The SAPO and the ALPO can have the same or different structure types. ALPO""s crystal structure allows oxygenates to have increased access to the pores of the SAPO during olefin conversion, in comparison to amorphous (non-molecular sieve) binders.
The oxygenate feedstock is preferably selected from the group consisting of methanol; ethanol; n-propanol; isopropanol; C4-C20 alcohols; methyl ethyl ether; dimethyl ether; diethyl ether; di-isopropyl ether; formaldehyde; dimethyl carbonate; dimethyl ketone; acetic acid; and mixtures thereof More preferably, the oxygenate feedstock is methanol, dimethyl ether or a mixture thereof
In order to convert the oxygenate to olefin product, the process is preferably performed at a temperature between 200xc2x0 C. and 800xc2x0 C.